The invention relates to the measurement of short chain branching in an ethylene 1-olefin copolymer as a function of its molecular weight. More particularly, the invention relates to such a measurement carried out by combining size exclusion chromatography, infrared (such as Fourier transform-infrared—“FT-IR”) spectrophotometry, and chemometric analysis.                One property of synthetic polymers, such as olefin copolymers, is that these macromolecules have a molecular weight distribution—some of the polymer chains are longer than others.        
An olefin copolymer also can be characterized by its degree of short-chain branching. The degree of short-chain branching can be determined by determining the number of methyl groups per 1000 carbon atoms in the sample. Given the average molecular weight of the sample, the number of methyl groups attributable to the ends of the polymer backbones can be calculated and subtracted from the number of methyl groups per 1000 carbon atoms to determine the number of methyl groups resulting from side branching. Each n-alkyl side chain has one methyl group.
In addition, an olefin copolymer can be characterized by the degree of short-chain branching as a function of its molecular weight distribution. In other words, a polymer can be characterized according to how many side chains are present on low-molecular weight polymer chains versus high-molecular weight polymer chains in a single bulk copolymer sample.
Information about the degree of short-chain branching (“SCB”) of an olefin copolymer, expressed as a function of the molecular weight distribution (“MWD”) of the copolymer, is useful for optimizing various properties of the olefin copolymer. Short-chain branching of the copolymer as a function of its molecular weight distribution affects such properties as the density, solvent extractables, and stress crack resistance of olefin copolymers. With this short-chain branching information in hand, the resin designer can modify the olefin copolymer polymerization process to optimize these properties of the resulting copolymer product.
The conventional analysis of short chain branching in an ethylene 1-olefin copolymer as a function of its molecular weight distribution involves solvent fractionation and subsequent characterization by NMR spectroscopy. Although the resulting values for short-chain branching distribution (“SCBD”) are highly accurate, gathering the wanted information is labor and time intensive.
Chemometric analysis is a multivariate statistical technique of mathematically treating data from a plurality of measurements to improve the selectivity of the analytical results. See, for example, Stetter, J. R., Jurs, P. C., and Rose, S. L., Anal. Chem. Vol. 58, pp. 860–866 (1986), cited in U.S. Pat. No. 4,874,500. Also, see the text Chemometrics-a Practical Guide, by K. R. Beebe, R. J.
Pell, and M. B. Seasholtz, Wiley, New York, 1998.
The inventors are not aware that chemometric analysis has been used to assist the determination of the degree of short-chain branching in a sample as a function of its molecular weight.
U.S. Pat. No. 5,700,895 (the '895 patent) discloses a method to measure the coefficient of variation of chemical composition distribution, Cx, and claims an ethylene-α-olefin copolymer having Cx of 0.40 to 0.80 among five parameter limitations. The method (column 12, lines 30–67 & column 13, lines 1–37) includes FT-IR measurement of temperature rising elution fractions (“TREF” fractions) at each of 39 temperatures in the range −10 to 145° C. Chemical composition distribution, i.e. short chain branching obtained from spectral peak areas over the interval from 2983 to 2816 cm−1 (SCBi), is plotted as a function of elution temperature.
The '895 patent does not disclose using size-exclusion chromatography (“SEC”) for fractionation or chemometric analysis for comparison of FT-IR curves. A TREF analysis involves the separation of a sample into fractions based on their differences in solubility in a solvent at different temperatures. Since both the molecular weight of a fraction and its degree of branching have impacts on its solubility, this technique does not separate the respective contributions of these two factors. This technique thus does not allow one to determine the degree of branching as a function of molecular weight. This technique also does not provide information on the statistical error of the results. This technique is also laborious.
U.S. Pat. No. 5,039,614 (the '614 patent) discloses a method to form solute films from solutions originating from fractionation based on a combination of physical and chemical property differences of ethylene/propylene copolymers (see claims 1 & 6 of the '614 patent). The films are obtained by rapid evaporation of solvent fractions from a gel permeation chromatography (“GPC”) column. FT-IR data on the films characterizes the composition distribution of each polymer fraction. This is exemplified for two ethylene/propylene copolymer resin (“EPR”) samples (column 8, lines 19–20). Chemometric analysis is neither suggested nor disclosed. The '614 patent discloses the formation of a polymer film by solvent evaporation before use of FT-IR to measure co-monomer incorporation.
U.S. Pat. No. 5,151,474 discloses and claims an ethylene polymerization process control method that uses FT-IR or other methods (column 4, lines 3–51) and chemometric analysis (column 4, lines 52–57) to measure the proportion of ethylene and 1-octene in a heptane solvent. There is no suggestion of polymer fractionation or a branching measurement.
Blitz, J. P. & McFaddin, D. C., “Characterization of Short Chain Branching in Polyethylene Using Fourier Transform Infrared Spectroscopy”, J. APPL. POLYM. SCI. 1994, 51, 13–20, discloses the use of methyl and methylene rocking bands in the infrared spectrum to distinguish and quantify methyl, ethyl, butyl, hexyl and isobutyl branches in linear low-density polyethylene (“LLDPE”). There is no suggestion of polymer fractionation.
Eric T. Hsieh, Chung C. Tso, Jim Dyers, Timothy W. Johnson, Qiang Fu, and Stephen Z. D. Cheng, “Intermolecular Structural Homogeneity of Metallocene Polyethylene Copolymers,” J. MACROMOL. SCI.-PHys. B36(5), 615–628 (1997) discloses measurement of SCB distribution of polymer blends using cross fractionation and 13C NMR (carbon-13 nuclear magnetic resonance).
The conventional methods for fractionating polyolefins are laborious and time-consuming. For example, a single typical cross-fractionation analysis may require 40 to 50 different samples to be processed. Because each sample requires a minimum of 24 hours to process, just the separation step alone requires at least 40 days. Furthermore, an additional 24 hours is needed to analyze each sample by NMR, thereby requiring another 40 days to complete the analysis. The cross-fractionation technique also has the disadvantage of not providing a determination of the statistical error arising from the analysis, as a function of polymer chain length.